1. Field of the Invention
This invention relates generally to a plastic exhaust system for a fuel cell vehicle and, more particularly, to a plastic exhaust system for a fuel cell vehicle, where the exhaust system includes a conductive layer to dissipate electro-static charge.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines. Fuel cell vehicles are expected to rapidly increase in popularity in the near future in the automotive marketplace.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. In one known type of fuel cell system, the hydrogen gas fuel is injected into the anode side of the fuel cell stack by one or more injectors. The injector controls the amount of injected fuel for a particular stack current density based on pulse width modulation (PWM) control signal that controls the opening and closing of the injector.
Typically, hydrogen gas for the fuel cell system is stored in a high pressure storage tank system including one or more interconnected pressure vessels on the vehicle to provide the hydrogen gas necessary for the fuel cell stack. The pressure within the vessels can be 700 bar or more. In one known design, the pressure vessels include an inner plastic liner that provides a gas tight seal for the hydrogen gas, and an outer carbon fiber composite layer that provides the structural integrity of the vessel.
Unlike the exhaust of an internal combustion engine, the exhaust from a fuel cell stack on a vehicle is warm, but not hot, and thus the exhaust system of a fuel cell vehicle does not need to be configured of metal components. Therefore, it has been proposed in the art to configure the exhaust system of a fuel cell vehicle using plastic components to reduce the weight of the vehicle, reduce vehicle cost, etc. Further, because the exhaust from the fuel cell stack is warm, the exhaust pipes can be closely mounted to the underbody of the vehicle chasse without concern for damaging vehicle parts or heating the passenger compartment of the vehicle, as opposed to internal combustion engines which require a space between the exhaust gas line and the vehicle underbody. A representative plastic exhaust system for a fuel cell vehicle is disclosed in U.S. patent application Ser. No. 13/214,777, titled Underbody Integrated Exhaust Path for Fuel Cell Vehicles, filed Aug. 27, 2011, assigned to the assignee of this application, and herein incorporated by reference.
Because the proposed plastic exhaust system for a fuel cell vehicle is to be made of a plastic material, it will have a low electrical conductivity. Thus, the water vapor and other exhaust materials that flow through the exhaust system will cause an electro-static charge build-up on the exhaust system that will not be significantly dissipated because of the non-conductive material. Once enough static charge builds up in the exhaust system, arcing could occur from the area on the exhaust system having the electro-static charge build-up to a conductive part on the chasse of the vehicle representing vehicle ground. Because the fuel cell stack exhaust may include some amount of hydrogen gas, which is highly flammable, this arcing potentially could ignite the hydrogen gas, which has obvious undesirable implications.